xfs: factor dir2 free block reading

Also factor out the updating of the free block when removing entries
from leaf blocks, and add a verifier callback for reads.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Phil White <pwhite@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

diff --cc fs/xfs/xfs_dir2_leaf.c
Simple merge

diff --cc fs/xfs/xfs_dir2_node.c
Simple merge

diff --cc fs/xfs/xfs_dir2_priv.h
Simple merge